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The Genetics of Viruses and Bacteria Chapter 18 Microbial Model Systems •Recall that bacteria are prokaryotes –With cells much smaller and more simply organized than those of eukaryotes •Viruses Virus –Are smaller and simpler still Bacterium Animal cell Animal cell nucleus 0.25 m Figure 18.2 Viruses •Viruses called bacteriophages –Can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli Figure 18.1 0.5 m Obligate Intracellular Parasites •A virus has a genome but can reproduce only within a host cell The Discovery of Viruses: Scientific Inquiry •Tobacco mosaic disease –Stunts the growth of tobacco plants and gives their leaves a mosaic coloration Figure 18.3 TMV •In the late 1800s –Researchers hypothesized that a particle smaller than bacteria caused tobacco mosaic disease •In 1935, Wendell Stanley –Confirmed this hypothesis when he crystallized the infectious particle, now known as tobacco mosaic virus (TMV) Viruses –Are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope •Viral genomes may consist of –Double- or single-stranded DNA –Double- or single-stranded RNA Capsids and Envelopes •A capsid –Is the protein shell that encloses the viral genome RNA Capsomere DNA Glycoprotein 70–90 nm (diameter) 18 250 mm CAPSID 20 nm TMV & 50 nm (b) Adenoviruses Adenovirus Capsids and Envelopes Envelopes –Membranous coverings derived from the membrane of the host cell Viral Envelopes •Many animal viruses –Have a membranous envelope •Viral glycoproteins on the envelope –Bind to specific receptor molecules on the surface of a host cell Bacteriophages A.K.A. phages –Have the most complex capsids found among viruses General Features of Viral Reproductive Cycles •Viruses are obligate intracellular parasites –They can reproduce only within a host cell •Each virus has a host range –A limited number of host cells that it can infect General Features of Viral Reproductive Cycles •Viruses use enzymes, ribosomes, and small molecules of host cells to synthesize progeny viruses DNA Capsid VIRUS HOST CELL Viral DNA mRNA Viral DNA Capsid proteins Viral Reproductive Mechanisms –Lytic cycle Is a phage reproductive cycle that culminates in the death (lysis) of the host –Lysogenic cycle Replicates the phage genome without destroying the host Lytic Cycle (Viral Reproduction) DOCKING with the host receptor protein PENETRATION of the viral nucleic acid into the host cytoplasm (Restriction Endonucleases, A.K.A. restriction enzymes break up host DNA) BIOSYNTHESIS of the viral components Assembly (MATURATION) of the viral components into complete viral units RELEASE of the completed virus from the host cell Phage assembly Head Tails Tail fibers Lysogenic Cycle Lambda •Temperate phages Are capable of using both the lytic & lysogenic cycles of reproduction Prophage When viral DNA is integrated into the bacterial chromosome (Plasmid) Capsid RNA Envelope (with glycoproteins) HOST CELL Viral genome (RNA) Template mRNA Capsid proteins ER Glycoproteins Copy of genome (RNA) •Retroviruses, such as HIV, use the enzyme reverse transcriptase –To copy their RNA genome into DNA, which can then be integrated into the host genome as a provirus (Integrates into host DNA) Glycoprotein 2 of each Capsid Reverse transcriptase HIV Viral envelope RNA (two identical strands) Evolution of Viruses •Viruses do not really fit our definition of living organisms since viruses can reproduce only within cells –They probably evolved after the first cells appeared, perhaps packaged as fragments of cellular nucleic acid Viral Diseases in Animals •Viruses may damage or kill cells –By causing the release of hydrolytic enzymes from lysosomes •Some viruses cause infected cells –To produce toxins that lead to disease symptoms Viral Diseases in Animals •Viruses may damage or kill cells (Amount of damage depends on the ability of infected tissue to regenerate by mitosis) -Respiratory tract epithelium repairs quickly from adenovirus infection - Nerve tracts affected by polio virus is permanent •Find host cells using “lock & key” fit with proteins on virus & host cell receptors Prions Protein infectious particles Contain no RNA or DNA Long incubation period (~10 years) Prions –Are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals –Propagate by converting normal proteins into the prion version Prion Original prion Many prions Normal protein Prions New prion Emerging Viruses Are those that appear suddenly or suddenly come to the attention of medical scientists 3 processes contribute to emerging viruses 1. Mutation of existing viruses as RNA is not corrected by proofreading e.g. SARS 2. Spread from one host species to another e.g. Hanta Virus 3. Dissemination from a small isolated population e.g. HIV SARS – Severe Acute Respiratory Syndrome (b) The SARS-causing agent is a coronavirus (a) Young ballet students in Hong Kong like this one (colorized TEM), so named for the wear face masks to protect themselves “corona” of glycoprotein spikes protruding from from the virus causing SARS. the envelope. Figure 18.11 A, B Emerging Viruses are NOT new They are existing viruses that •Mutate •Spread to new host species •Disseminate more widely in the host species Small Pox Polio Polio Herpes Simplex Hepatitis Varicella Zoster Mumps Measles - Rubeola Other Viruses that affect humans •Influenza Virus •Rubella •Parvo-virus •Epstein Barr Virus •Hanta Virus •(HPV) Human Papilloma Virus •(RSV) Respiratory Syncytial Virus •Rabies •Rhinovirus •Rotavirus •West Nile Virus Viral Diseases in Plants •More than 2,000 types of viral diseases of plants are known •Common symptoms of viral infection include –Spots on leaves and fruits, stunted growth, and damaged flowers or roots Viral Diseases in Plants •Plant viruses spread disease in two major modes –Horizontal transmission, entering through damaged cell walls –Vertical transmission, inheriting the virus from a parent Viroids -The Simplest Infectious Agent –Are circular RNA molecules that infect plants and disrupt their growth Bacteria •Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria •Bacteria allow researchers –To investigate molecular genetics in the simplest true organisms The Bacterial Genome and Its Replication •The bacterial chromosome –Is usually a circular DNA molecule with few associated proteins •In addition to the chromosome –Many bacteria have plasmids, smaller circular DNA molecules that can replicate independently of the bacterial chromosome The Bacterial Genome and Its Replication •Bacterial cells divide by binary fission –Which is preceded by replication of the bacterial chromosome Replication fork Origin of replication Termination of replication Binary Fission Mutation and Genetic Recombination as Sources of Genetic Variation •Since bacteria can reproduce rapidly –New mutations can quickly increase a population’s genetic diversity •Genetic diversity –Can also arise by recombination of the DNA from two different bacterial cells –Remember that prokaryotes don’t undergo meiosis or fertilization Recombination in Bacteria •Three processes bring bacterial DNA from different individuals together –Transformation –Transduction –Conjugation Transformation Is the alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment Transduction Phages carry bacterial genes from one host cell to another Conjugation and Plasmids •Conjugation –Is the direct transfer of genetic material between bacterial cells that are temporarily joined DNA transfer is one way Figure 18.17 Sex pilus 1 m The F Plasmid and Conjugation •Cells containing the F plasmid, designated F+ cells –Function as DNA donors during conjugation –Transfer plasmid DNA to an F recipient cell F Plasmid Bacterial chromosome F+ cell F+ cell Mating bridge F– cell 2 1 A cell carrying an F plasmid (an F+ cell) can form a mating bridge with an F– cell and transfer its F plasmid. Figure 18.18a F+ cell Bacterial chromosome 3 A single strand of the F plasmid breaks at a specific point (tip of blue arrowhead) and begins to move into the recipient cell. As transfer continues, the donor plasmid rotates (red arrow). 4 DNA replication occurs in both donor and recipient cells, using the single parental strands of the F plasmid as templates to synthesize complementary strands. The plasmid in the recipient cell circularizes. Transfer and replication result in a compete F plasmid in each cell. Thus, both cells are now F+. F Plasmid recombination Chromosomal genes can be transferred during conjugation when the donor cell’s F factor is integrated into the chromosome Hfr cell A cell with the F factor built into its chromosome The F factor of an Hfr cell Brings some chromosomal DNA along with it when it is transferred to an F– cell R plasmids and Antibiotic Resistance Confer resistance to various antibiotics Transposition of Genetic Elements •Transposable elements –Can move around within a cell’s genome –Are often called “jumping genes” –Contribute to genetic shuffling in bacteria by folding the DNA Insertion Sequences •An insertion sequence contains a single gene for transposase –An enzyme that catalyzes movement of the insertion sequence from one site to another within the genome Insertion sequence 3 A T C C G G T… A C C G G A T… 3 5 TAG G C CA… TG G C CTA… 5 Transposase gene Inverted Inverted repeat repeat (a) Insertion sequences, the simplest transposable elements in bacteria, contain a single gene that encodes transposase, which catalyzes movement within the genome. The inverted repeats are backward, upside-down versions of each other; only a portion is shown. The inverted repeat sequence varies from one type of insertion sequence to another. Figure 18.19a Transposons •Bacterial transposons –Also move about within the bacterial genome –Have additional genes, such as those for antibiotic resistance Transposon Insertion sequence Antibiotic resistance gene Insertion sequence 5 5 3 3 Inverted repeats Transposase gene (b) Transposons contain one or more genes in addition to the transposase gene. In the transposon shown here, a gene for resistance to an antibiotic is located between twin insertion sequences. The gene for antibiotic resistance is carried along as part of the transposon when the transposon is inserted at a new site in the genome. Figure 18.19b Prokaryotic Gene Expression •Individual bacteria respond to environmental change by regulating their gene expression •E. coli, a type of bacteria that lives in the human colon –Can tune its metabolism to the changing environment and food sources Response to the environment •This metabolic control occurs on two levels –Adjusting the activity of metabolic enzymes already present –Regulating the genes encoding the metabolic enzymes (a) Regulation of enzyme activity Precursor Feedback Inhibition Feedback inhibition (b) Regulation of enzyme production Enzyme 1 Gene 1 Enzyme 2 Gene 2 Enzyme 3 Gene 3 Regulation of gene expression -– Enzyme 4 Gene 4 – Enzyme 5 Tryptophan Figure 18.20a, b Gene 5 Operons: The Basic Concept •In bacteria, genes are often clustered into operons, composed of –An operator, an “on-off” switch –A promoter –Genes for metabolic enzymes •An operon –Is usually turned “on” –Can be switched off by a protein called a repressor Operon Parts •The regulatory gene codes for the repressor protein. •The promoter site is the attachment site for RNA polymerates. •The operator site is the attachment site for the repressor protein. •The structural genes code for the proteins. Operon Parts •The repressor protein is different for each operon and is custom fit to the regulatory metabolite. Whether or not the repressor protein can bind to the operator site is determined by the type of operon. •The regulatory metabolite is either the product of the reaction or the reactant depending on the type of operon. The trp operon: regulated synthesis of repressible enzymes trp operon Promoter DNA Promoter Genes of operon trpD trpC trpE trpR Regulatory gene mRNA 5 3 Operator Start codon RNA polymerasemRNA 5 Inactive repressor trpA Stop codon E Protein trpB D C B A Polypeptides that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon’s genes. Figure 18.21a DNA No RNA made mRNA Protein Active repressor Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off. As tryptophan accumulates, it inhibits its own production by activating the repressor protein. Figure 18.21b http://bcs.whfreeman.com/thelifewire/content/chp13/1302002.html Trp Operon Repressible and Inducible Operons: Two Types of Negative Gene Regulation •In a repressible operon –Binding of a specific repressor protein to the operator shuts off transcription (Found in anabolic pathways) http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120080/bio26.swf::The%20Tryptophan%20Repressor •In an inducible operon –Binding of an inducer to an innately inactive repressor inactivates the repressor and turns on transcription (Found in catabolic pathways) Lac Operon The lac operon: regulated synthesis of inducible enzymes http://www.sumanasinc.com/webcontent/animations/content/lacoperon.html Promoter Regulatory gene DNA Operator lacl lacZ 3 mRNA Protein No RNA made RNA polymerase 5 Active repressor (a) Lactose absent, repressor active, operon off. The lac repressor is innately active, and in the absence of lactose it switches off the operon by binding to the operator. Figure 18.22a Lac Operon “off” lac operon DNA lacz lacl 3 mRNA 5 lacA RNA polymerase mRNA 5' 5 mRNA -Galactosidase Protein Allolactose (inducer) lacY Permease Transacetylase Inactive repressor (b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced. Figure 18.22b Lac Operon “on” Types of Operons Inducible enzymes Usually function in catabolic pathways Repressible enzymes Usually function in anabolic pathways